1. Field of the Invention
The present invention generally relates to an integrated circuit, and more particularly to an electrostatic discharge (ESD) protection circuit.
2. Description of Related Art
ESD is a phenomenon of a sudden transfer of electrostatic charge between bodies at different electrostatic potentials. Semiconductor devices and circuits in ICs can be damaged by ESD. For example, a charge carrying object, such as a human body walking on a carpet, a machine for packaging ICs or an apparatus for testing ICs, can discharge to the IC chip when they are in contact with each other. Thus, the IC chip is damaged or broken by the transient power of ESD.
To protect the IC chip from being damaged by ESD, an ESD protection device is added to the IC design. Generally, there are many kinds of designs for ESD protection devices. A typical one is a transistor layout with a metal-oxide-semiconductor (MOS) transistor to achieve the purpose of ESD protection, in which an ESD current can be released by turning on a parasitic bipolar junction transistor generated below the MOS transistor. Further, a substrate bias is increased by applying a substrate triggering technology.
In designing the layout for such an ESD protection device, since resistance is proportional to voltage, a well, a shallow trench isolation (STI), or a field oxide (FOX) are often disposed to increase the substrate resistance, so that the substrate bias is increased accordingly, and the ESD protection ability is improved. However, the above-mentioned method also causes an unstable substrate voltage for the ESD protection device in normal operation, so that the operation performance of the ESD protection device is affected. Further, when the ESD current occurs, the ESD protection ability is affected as a consequence of the incomplete turning on if the layout is not designed or arranged appropriately.
Accordingly, it has become an important topic in the industry about how to design an appropriate ESD protection circuit without disturbing the normal operation of the device.